rockshox reverb sizing advice

cmachine Posts: 84
edited August 2014 in MTB buying advice
Hi all. Need some advice from someone a bit more knowledgeable than me. I have a Giant Anthem X4, Large size and I'm 6' 1" tall. I want to get a Rockshox Reverb but am a bit unsure about sizes. I know I need a 30.9 and I want the 125mm drop....just unsure between the 380mm or the 420mm. My existing Giant Connect seatpost is 375mm. I have it extended to around the 30 mark which means roughly 220mm is extended from collar to seatpost rail and about 155 below the seatpost not dangerously close to the min. insertion point. What I've noticed that I cannot bring my existing seatpost all the way down to the seatclamp collar. This is probably normal given the rear sus linkages in the seatpost tube.

What I am confused about is would the 420mm give me any benefit or because there is more of it would it mean I couldn't drop the Reverb lower? I know the drop is always 125mm but would a 420 limit me as there is more of it to try and fit into the seatpost tube. I've tried to explain as best as I can. cheers.
2006 Specialized Hardrock Disc (rebuilt)
2011 BeOne Karma Hardtail (disassembled!)
2012 Giant Anthem X4